This invention relates generally to a reversible rotary actuator and specifically to an actuator having an electric motor which is selectively operable to rotate an output shaft in one direction. During driving of the output shaft by the motor, a torsion spring is wound so as to store energy for rotating the shaft in the other direction when the motor is de-energized and the spring unwinds.
More particularly, the invention relates to a rotary actuator of the type in which the motor rotates the output shaft and winds the spring by way of a gear train which substantially reduces the speed and substantially amplifies the torque of the motor. When the spring unwinds to rotate the output shaft, the spring acts reversely through the gear train and backdrives the motor shaft.
An actuator of this type is frequently used to drive a utilization device such as a smoke and fire damper in the duct of a heating, ventilating and cooling system. When the motor is de-energized, the spring drives the output shaft in a direction moving the damper to a closed position against a fixed stop. During driving of the output shaft by the spring the gear train and the motor shaft are accelerated and develop substantial kinetic energy. When the damper is abruptly stopped, the gear train and the motor shaft are subjected to impact loading unless the kinetic energy is dissipated. In prior actuators of this type, friction clutches have been used to dissipate the kinetic energy as heat. Such clutches, however, are relatively complex and expensive and substantially increase the cost of a comparatively small and low torque actuator.